That's a life lesson he'll never forget, probably scare the crap out of him (which isn't a bad thing). There are some rivers I fish you've gotta watch every single step you take or else you're going for a dip. I'd probably look at what were on his feet when he slipped. Running shoes? Hiking boots? A cheap pair of hiking boots with a few sheet metal screws in the bottom will make a world of difference on slippery algae. I think the key here is prevention, a life jacket or PFD is great, but let's not even get to that point next time. Perhaps even a old ski pole to use as a wading staff until he gets more comfortable around that type of water.